The present invention relates generally to an oxygen sensor for detecting the oxygen concentration of a measurement gas, especially an exhaust gas produced by an internal combustion engine. More particularly, the invention is concerned with such an oxygen sensor which uses an elongate planar sensing element having an oxygen detecting portion at its one end, and which is simple in construction and easy to manufacture and assemble.
There has been known an oxygen sensor which detects or determines the oxygen concentration of an exhaust gas emitted from internal combustion engines, for the purpose of controlling the combustion or fuel burning condition of the engine according to signals produced by the oxygen sensor, and thereby purifying the exhaust gas and saving the fuel consumption of the engine. An example of such oxygen sensors uses a sensing element which comprises a body of oxygen-ion conductive solid electrolyte such as zirconium oxide doped with calcium oxide or yttrium oxide, and further comprises suitable electrodes disposed on opposite surfaces of the solid electrolyte body. In this oxygen sensor, one of the electrodes is exposed to a reference gas while the other electrode is exposed to the exhaust gas. In operation, the oxygen sensor produces an output signal which represents an electromotive force induced between the two electrodes according to the principle of an oxygen concentration cell. In recent years, there has been an increasing tendency to use an elongate planar sensing element rather than a conventionally used tubular sensing element, in view of ease of manufacture and structural simplicity of the sensor. Such an elongate planar sensing element has, at its one end, a detecting portion to be exposed to an exhaust gas or other measurement gas.
In an oxygen sensor discussed above, the output of the sensor is obtained as an electric signal which represents the oxygen partial pressure of the measurement gas detected by the detecting portion. This electric signal should be fed to an external device. For this purpose, electrical leads of the electrodes should be electrically connected to external conductor members or lead wires which are inserted into the housing of the oxygen sensor. Since the sensing element carrying the leads is of elongate planar configuration, it has been impossible to connect the external lead wires directly to the leads on the sensing element. Accordingly, it has been a common practice to use auxiliary leads which are extensions of the leads formed on the sensing element. These auxiliary leads are connected to the external lead wires as with suitable calking actions, at a position away from the sensing element. However, the use of such auxiliary leads complicates the electrical arrangement of the sensor, and therefore results in reduced efficiency of manufacture and assembly of the sensor. Further, the procedure to make electrical connections between the auxiliary leads and the external lead wires is difficult and cumbersome. In addition, the reliability of the electrical connections is not completely satisfactory.
It is also noted that an oxygen sensor of the aforementioned type is provided with an electrical heater which is incorporated within the sensing element in order to heat the detecting portion for reliable and accurate operation of the detecting portion. To supply power to such a heater, additional external lead wires should be connected to the leads of the heater. In this case, too, the same problems as pointed out above are encountered. In summary, a plurality of conductor members, i.e., plural pairs of lead wires should be connected to the corresponding plural pairs of electrical leads of the sensing element, within the sensor housing, to obtain an output signal from the detecting portion of the sensing element and to actuate the heater in the element. Considering the need that the oxygen sensor be constructed as compact as possible, it is an extremely difficult procedure to make electrical connections of the signal leads and the heater leads to the corresponding external wires, at not a few locations within the compact assembly of the oxygen sensor. Even if these electrical connections are not impossible, the provisions for the connections necessarily complicate the overall structure of the oxygen sensor.